Musannaf Ibn Abi Shayba 21:40

Abu Bakr narrated to us, he said: Ubaydah ibn Humayd narrated to us, from Humayd al-Tawil, from Talhah, from Umm al-Darda’, who said: "The supplication of a Muslim person for his brother while he is absent is not rejected." She said: And she said: "At his side is an angel; he does not supplicate for him with good except that the angel says: 'And for you [the same].'"